ACF PRO is the premium version of Advanced Custom Fields (ACF), a plugin for WordPress from WP Engine. A custom field is an extra piece of structured data stored with a post, user or other object, such as a sub-heading or a hero image. ACF adds a field builder to the WordPress edit screens; a developer creates a field group, adds fields and sets location rules that decide where the group appears.
Theme templates read the saved values with PHP functions such as get_field() and the_field(). ACF has over 30 field types. The PRO version adds the Repeater field (a set of sub fields that can repeat), the Flexible Content field (a layout manager for groups of sub fields), the Gallery and Clone fields, options pages for global data, and ACF Blocks, a PHP framework for custom blocks in the WordPress block editor.
ACF PRO is a stand-alone plugin sold as a yearly licence per number of websites; the licence key enables updates and the PRO features.